1. Field of the Invention
The invention relates to apparatus for preventing the tilting of telescopic jib cranes, especially mobile cranes.
2. Prior Art
The slewing ring of a known mobile crane, together with the frame of the lifting appliance, is mounted on the chassis of the vehicle. The jib is swung upwards about a pivot point by means of a hydraulic cylinder, hereinafter called the "jib-raising cylinder". A further hydraulic cylinder extends and retracts the telescopic jib and is hereinafter called the "telescope cylinder". A winch is used for lifting and lowering the load. Finally, the slewing ring may also be rotated.
When the mobile crane is in use it is stabilized by lateral props which limit the extent to which the crane would tilt if overloaded. To ensure safety during operation of the crane, it is laid down that the return moment counteracting the tilting moment must always be at least 1.4 times the tilting moment. The tilting moment is the product of the load and its leverage. The leverage of the load is at its greatest when the jib is fully lowered, the telescope is extended and the jib is at right angles to the longitudinal axis of the vehicle. Any other movement of the crane reduces the leverage of the arm. The weight of the load which can still be lifted by the crane increases correspondingly.
Various systems are known for limiting the load in dependence upon the position of the various components. In one proposal, the trigonometrical interdependence of the various factors which determine the moment is represented by a cam-plate or the like by which the contacts of an electrical safety switching arrangement are controlled; in this system the magnitude of the load is converted by means functioning in proportion to the load, for example, by a set of laminated spring elements or the like (see "Die Berufsgenossenschaft", No. 10/11, 1958, pages 1-16).
The main disadvantage of the previously proposed safety means resides in the complicated nature of the components thereof and thus in the possibility of their breaking down.